Oxidation products, citral

Table 3 indicates that 5%Pt,l%Bi/C is active for three reaction cycles in the selective oxidation of the chosen alcohols. For primary alcohols the use of water as solvent can promote the aldehyde to carboxylic acid reaction (3). This effect is observed in the selective oxidation of 1-octanol where octanoic acid is formed with 97% selectivity in the first cycle dropping to 81% in the third. In the selective oxidation of geraniol only citral is observed as the oxidation product. The presence of the double bond stabilises the aldehyde even in the presence of... 

In the presence of oxygen, ketyl radicals and those derived from their hydrogen donors are converted to peroxy radicals. Figure 6.15 shows an example of an aldehyde photolyzed in ethanol. These peroxy radicals are often quite susceptible to loss of HOO and subsequent production of hydrogen peroxide (see Equations 4.10 and 4.23 in the Oxidation chapter). Citral (25), a naturally occurring monoterpene aldehyde that is abundant in Citrus species and other plants, has been shown to be phototoxic, perhaps by this mechanism (Asthana et al., 1992). 

Three types of volatile products were found to be obtained from citral (neral (10) and geranial (12)) after UV light irradiation at pH 3.5. Aldehydes 3 and 4, photocitral B (5), aldehyde 6, and photocitral A (7) were formed through photochemical cyclization (12-13) (Table I and Figure 1). Both tetrahydrofurans 16 and 17 were obtained by oxidation (4) (Table I). The acid-catalyzed cyclia tion products were furan 2 and alcohols 13-15, 19 and 21 (2, 24-27) (Table I and Figure 2). As shown in Table II, the relative amount of the acid-catalyzed cyclization products 2, 13-15, 19, and 21 was extremely high (83%) under dark conditions. However, the UV light irradiation increased the photochemical cyclization products 3-7 and oxidation products 16 and 17. The amount of products of the former and that of the latter were 38% and 27%, respectively. 

In order to clarify the formation pathway of oxidation products 16 and 17 from citral (10 and 12), the effect of UV light, and AAPH, that is, radical initiator, under acidic or neutral conditions was studied (Table IV). The oxidation products 16 and 17 also increased using AAPH instead of UV light under acidic conditions. However, under neutral conditions, epoxides 40 and 41, the intermediates, were obtained instead of 16 and 17 (28-29). The epoxides 40 and 41 were converted into 16 and 17 by the furan-ring formation under acidic conditions. Therefore, as shown in Figure 7, there seemed to be the alternative pathway via epoxidation, hydration, and cyclization compared with Grein s report (4). 

Figure 6. The yield of oxidation products from citral (10 and 12) vs. DPPH radical scavenging activity of each additive compound at pH 3.5.

Figure 7. Formation of fur an 16 and 17, the oxidation products, from citral (10...

Table IV. Effect of UV Lighf and AAPH under Acidic or Neutrai Conditions on the Formation of Oxidation Products from Citral (10 and 12)...

In functional products, citral is subject to both acid and base reactions and to oxidation by air or by other components, such as hypochlorite bleach. Consequently, a great deal of research has been invested in the search for materials with the odor of citral, but with much better stability. 

In Part A geraniol is oxidized to geranial (citral) by Swern s modification of the Moffat oxidation. 1 The stereoisomerlc purity of the product is at least 98%. This procedure is readily conducted on a large-scale and requires only 4 hours time including distillation of oxalyl chloride. The oxidation of geraniol to pure (E)-geranial may also be accomplished by... 

Another example of cross-aldol condensation is the reaction between citral and acetone, which yields pseudoionone, an intermediate in the production of vitamin A. Noda et a/.[56] working at 398 K with a 1 1 molar ratio of reagents and 2 wt % of catalyst, obtained high conversions (98 %) with selectivities to pseudoionone close to 70 % with CaO and an Al-Mg mixed oxide catalyst these pseudoionone yields are greater than those reported for the homogeneous reaction. MgO exhibited poor activity, and under these conditions only 20 % citral conversion was obtained after 4 h in a batch reactor. Nevertheless, Climent et a/./571 working with 16 wt % MgO as a catalyst, a molar ratio of acetone to citral close to 3 and at 333 K, achieved 99 % conversion and 68 % selectivity to pseudoionone after 1 h. 

Two key intermediates in the production of vitamin A are citral and the so-called C5 aldehyde. In the modem routes to these intermediates, developed by BASF and Hoffmann-La Roche, catalytic technologies are used (see Fig. 2.29 and 2.30). Thus, in the synthesis of citral, the key intermediate is 2-methyl-l-butene-4-ol, formed by acid-catalyzed condensation of isobutene with formaldehyde. Air oxidation of this alcohol over a silver catalyst at 500°C (the same catalyst as is used for the oxidation of methanol to formaldehyde) affords the corresponding aldehyde. Isomerization of 2-methyl-l-butene-4-ol over a palladium-on-charcoal catalyst affords 2-methyl-2-butene-4-ol. The latter is then reacted with the aldehyde from the oxidation step to form an enol ether. Thermal Claisen rearrangement of the enol ether gives citral (see Fig. 2.29). 

Geraniol, Citral.—Citrene by oxidation yields alcohol and aldehyde products as follows ... 

Citral is the key odoriferous principle of lemon oil and is therefore potentially very useful in perfumery. However, it is not stable to oxidation and so cannot be used in functional products containing bleach. Since lemon is associated with cleanliness and freshness, this represents a serious challenge for household products. One of the solutions that has been found is to convert citral into the more stable nitrile, known as geranyl nitrile. Often, nitriles have odours that closely resemble the corresponding aldehyde, this being a case in point. Geranyl nitrile can be prepared from either citral or from methylheptenone, as shown in Scheme 4.15. 

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